1. Field of the Invention
The present invention relates to an improvement of the structure of a solid state electrolytic capacitor such as a tantalum solid state electrolytic capacitor and an aluminum solid state electrolytic capacitor.
2. Description of the Prior Art
Conventionally, solid state electrolytic capacitors of this type include, for example, a solid state electrolytic capacitor A described in Japanese Laid-open Patent Application No. S60-220922 and structured as shown in FIG. 1 and a solid state electrolytic capacitor B having a safety fuse described in Japanese Laid-open Patent Application No. H2-105513 and structured as shown in FIG. 2.
Specifically, the former solid state electrolytic capacitor A is formed in the following manner: A capacitor device 1 including a chip 1a, an anode bar 1b protruding from one end surface 1a' of the chip 1a and a cathode film 1c formed on the surface of the chip 1a except on the end surface 1a' is arranged between a pair of left and right lead terminals 2 and 3 by fixing the anode bar 1b of the capacitor device 1 to one lead terminal 2 and by directly connecting the cathode film 1c of the chip 1a of the capacitor device 1 to the other lead terminal 3, and then, the entire structure is encapsulated in a packaging member 4 such as a synthetic resin made mold.
The latter solid state electrolytic capacitor B having a safety fuse is formed in the following manner: a capacitor device 1 of a similar structure is arranged between a pair of left and right lead terminals 2 and 3' by fixing an anode bar 1b of the capacitor device 1 to the lead terminal 2 and by connecting a cathode film 1c of the chip 1a of the capacitor device 1 to the other lead terminal 3' by a safety fuse 5 such as a soldering wire which melts by an excess current or by an increase in temperature, and then, the entire structure is encapsulated in a packaging member 6 such as a synthetic resin made mold.
In the solid state electrolytic capacitors A and B, the lead terminals 2, 3 and 3' are formed to be of a surface mounted type so that they are mounted on a printed circuit board by bending them toward the bottom surfaces of the packaging members 4 and 6.
To manufacture the capacitor device 1 used for the solid state electrolytic capacitors A and B, the following method is employed: First, powder of a metal such as tantalum is shaped into a porous chip 1a so that the anode bar 1b protrudes from the end surface 1a' of the chip 1a, and then, the powder is sintered. Then, as shown in FIG. 3, the porous chip 1a is drenched in a chemical conversion solution C such as an aqueous solution of phosphoric acid so that the end surface 1a' of the chip 1a is at a depth H from the surface of the solution C, and a direct current is applied between the anode bar 1b and the chemical conversion solution C under this condition to cause anodic oxidation, so that a film D of a dielectric substance such as tantalum pentoxide is formed on the surface of the metal powder on the chip 1a and that a film D' of a dielectric substance such as tantalum pentoxide is formed at a portion corresponding to the length H on the periphery at the root of the anode bar 1b.
Then, as shown in FIG. 4, the chip 1a is drenched in an aqueous solution E of manganese nitrate until the solution E permeates into the chip 1a and then taken out from the solution E and sintered. By repeating this a plurality of times, a layer F of a solid state electrolyte such as manganese dioxide is formed on the surface of the film D of a dielectric substance such as tantalum pentoxide and the cathode film 1c made of a metal such as silver or nickel is formed on the surface of the chip 1a except on the end surface 1a' with a graphite film between.
In the process of manufacturing the capacitor device 1, when the layer F of a solid state electrolyte such as manganese dioxide is formed, the aqueous solution E of manganese nitrate permeates beyond the film D' of a dielectric substance such as tantalum pentoxide formed at the root of the anode bar 1b to above the film D' to form a layer F of a solid state electrolyte such as manganese dioxide there, so that the layer F of a solid state electrolyte such as manganese dioxide and the anode bar 1b electrically short-circuit. As a result, an inferior capacitor device is produced.
Conventionally, to reduce the production of inferior capacitor devices due to the over-percolation of the aqueous solution E of manganese nitrate in forming the layer F of a solid state electrolyte such as manganese dioxide, after the completion of the process of forming the film of a dielectric substance such as tantalum pentoxide, a synthetic resin G such as silicon resin is applied to the root of the anode bar 1b as shown in FIG. 5 or a ring member J made of a synthetic resin such as silicon resin is fixed to the root of the anode bar 1b as shown in FIG. 6.
In assembling the solid state electrolytic capacitors A and B shown in FIGS. 1 and 2 by using the capacitor device 1 thus manufactured, it is necessary for the lead terminal 2 to be away from the end surface 1a' of the chip 1a at a distance corresponding to a height T' or T" of the synthetic resin G or the ring member J provided at the root of the anode bar 1b where it is fixed to the end surface 1a'. In other words, a distance W' between the lead terminal 2 and the end surface 1a' of the chip 1a should be increased by an amount for providing the synthetic resin G or the ring member J at the root of the anode bar 1b. The distance W' is added to the total length L' of the solid state electrolytic capacitor.
Therefore, when the length L' of the solid state electrolytic capacitor A is predetermined, it is necessary to reduce the length, i.e. the volume of the chip 1a of the capacitor device 1 by an amount for providing the synthetic resin G or the ring member J. This leads to a decrease in capacitance of the solid state electrolytic capacitor. When the capacitance of the solid state electrolytic capacitor is predetermined, it is necessary to increase the length L' of the solid state electrolytic capacitor A by an amount for providing the synthetic resin G and the ring member J. This leads to an increase in size and weight of the solid state electrolytic capacitor.
In particular, in the solid state electrolytic capacitor B having a safety fuse shown in FIG. 2, it is also necessary to provide a considerable distance S' between the other lead terminal 3' and the chip 1a of the capacitor device 1 in order for the other lead terminal 3' not to be in contact with the cathode film 1c of the chip 1a, and the distance S' is added to the total length L". This leads to a decrease in capacitance and an increase in size and weight.